1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor storage element utilizing a ferroelectric material.
2. Description of the Related Art
A ferroelectric material has the property of spontaneous polarization and is characterized by reversing a direction of its spontaneous polarization in accordance with an applied electric field. There are two types of semiconductor storage elements in which spontaneous polarization of the ferroelectric material is utilized. One is of a so-called one-transistor type, and another is of a so-called one-transistor/one-capacitor type. There has recently been expected realization of the one-transistor type due to reasons that high-speed operation is possible, nondestructive readout of data can be effected, and high integration can be expected.
An example of the one-transistor type semiconductor storage element is disclosed in "Technical Report of IEICE SDM93-136, pp. 53-59". According to Metal/ Ferroelectric/ Metal/Insulator/Semiconductor (MFMIS) structure disclosed herein, an insulating film, a lower electrode, a ferroelectric film, and an upper electrode are formed in layers on a semiconductor substrate in that order. This structure is provided for the reason that the ferroelectric film does not satisfactorily grow on a semiconductor or an insulator. In the MFMIS structure, it suffices that the ferroelectric film (for example, a film of lead titanate-zirconate, or a film of bismuth strontium tantalate) may be formed on a conductor film (for example, a platinum (Pt) film), and therefore, film formation is facilitated.
Further, when voltage is applied to the ferroelectric film, charge is accumulated in the ferroelectric film itself due to residual polarization of the ferroelectric film. Specifically, when positive voltage is applied to the ferroelectric film, positive charge is accumulated therein. On the other hand, when negative voltage is applied to the ferroelectric film, negative charge is accumulated therein. In a semiconductor storage element having the MFMIS structure, the accumulated charge excites charge on the surface of a semiconductor substrate. Accordingly, even if applied voltage is 0 volt, a switching operation of a transistor, namely, on state or off state is selectively maintained. As a result, data is written in the ferroelectric film. Further, current flowing between a source electrode and a drain electrode varies in accordance with the switching state of a transistor, and therefore, readout of data is made possible by detecting the variation of current.
However, processing of the above-described ferroelectric film and conductor film cannot easily be carried out by reactive ion etching (RIE) normally used in a semiconductor process, and therefore, it is necessary to effect processing by milling using argon gas. In this method, however, selectivity with a gate oxide film cannot be obtained, and therefore, there is a possibility that a semiconductor (silicon substrate) be damaged, thereby interfering with an operation of a finished product.